Tissue cassettes are commonly used in histology laboratories to carry tissue samples while they are bathed in a series of washes designed to remove the water in the tissue and replace it with wax. The wax stiffens the tissue sufficiently to allow it to be thinly sliced in a microtome. The thin slice or section is then placed on a slide, stained and evaluated by a pathologist to diagnose potential disease.
Tissue processing includes a multitude of steps. Typically, the tissue is first dehydrated by replacing fixative and water with a dehydrating liquid. The tissue is then flushed of the dehydrating fluid using a fluid that is totally miscible with both the dehydrating fluid and an embedding medium. Impregnation follows by replacing the flushing (i.e., clearing) fluid with the embedding medium.
Impregnation is typically conducted in an automated, closed-door processor in which the tissue is first exposed to various solvents, e.g., a mixture of an alcohol and xylene, and then impregnated with the embedding media, e.g., a paraffin wax. This process often includes agitation of the materials, a vacuum or partial vacuum atmosphere, and warm temperatures, e.g., 60° C. or more for melted paraffin wax. The tissue samples are placed in tissue cassettes that allow for fluid exchange and that keep individual tissue samples separate throughout the process. Identification of each sample is critical and as such, each sample is individually labeled, preferably by bar code.
After impregnation excess wax (usually found about the edges of the cassette) is trimmed from the cassette either manually with a knife or by a heated plate. If the excess wax is trimmed, e.g., scrapped, from the cassette with a knife or similar instrument and the label contains one or more ripples, then the label can be inadvertently torn. The tissue is then removed from the cassette and placed in a mold. Hot wax is injected into the mold to surround the impregnated tissue. The tissue cassette is then placed on top of and in contact with the hot wax in the mold, and the impregnated sample attaches and fixes itself to the cassette by the hardening of the wax.
Histology laboratories vary greatly in size. Large laboratories will process between 400 and 1,000 cassettes per day. Small laboratories may only process one to 100 per day. The vast majority of small and medium size laboratories today mark their tissue cassettes by handwriting with a pencil or pen on a textured surface molded into the cassette for this purpose. Unfortunately, legibility is a major issue with hand marked cassettes. Additionally, the marking may be partially removed by the chemical washes during tissue processing.
Thermal transfer printed labels provide the desired legibility and, if properly formulated, the print, i.e., ink, can resist the harsh solvents used in tissue processing. For example, the print quality and solvent resistance of a thermal transfer printed label comprising a topcoat made from a composition as described in U.S. Pat. No. 7,081,284 carrying ink from a black 6400 series thermal transfer printer ribbon (available from Brady Worldwide, Inc.) is excellent. However, a mechanical means of attaching the label to the tissue cassette proved necessary due to a lack of a pressure sensitive adhesive (PSA) that can withstand the harsh solvents used in tissue processing. This mechanical attachment involves a form of “riveting” the label to the cassette by plunging a heated probe through the label (typically a multilayer structure of various plastic films) into the tissue cassette (typically made of a plastic, e.g., a polyacetal), melting the plastic of the cassette, and allowing the melted plastic to flow over at least a part of the label in a rivet-like formation (as more fully described in Published Patent Application No. 2008/0053605) to secure it to the cassette. Any number of rivets can be used to attach the label to the cassette but since most labels are rectangular in shape, typically rivets are placed at two of the four corners, or at all four corners.
One inconvenience of this mechanical attachment method is that hot melted wax can work its way under and solidify beneath the label. This, in turn, can cause the label to ripple which not only adversely affects the aesthetic appearance of the label, but can interfere with the ability to read a barcode if such information is printed on the label. Moreover and as earlier noted, the presence of a ripple during the trimming of excess wax from the cassette and lead to an unwanted tear in the label.
Accordingly, of interest is the design and manufacture of a label for a tissue cassette that will not, or only insignificantly, ripple during the process of attaching, e.g., staking, the label to a tissue cassette, or during the process in which the tissue sample within the cassette is processed.